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2 Journal of the Mechanics and Physics of Solids 56 (2008) Contents lists available at ScienceDirect Journal of the Mechanics and Physics of Solids journal homepage: Surface stress effects on the resonant properties of metal nanowires: The importance of finite deformation kinematics and the impact of the residual surface stress Harold S. Park a,, Patrick A. Klein b a Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA b Franklin Templeton Investments, San Mateo, CA 94403, USA article info Article history: Received 5 January 2008 Received in revised form 6 August 2008 Accepted 3 August 2008 Keywords: Nanowires Resonant frequency Surface stress Surface Cauchy Born Finite elements abstract We utilize the recently developed surface Cauchy Born model, which extends the standard Cauchy Born theory to account for surface stresses due to undercoordinated surface atoms, to study the coupled influence of boundary conditions and surface stresses on the resonant properties of h 0 0i gold nanowires with f 0 0g surfaces. There are two major purposes to the present work. First, we quantify, for the first time, variations in the nanowire resonant frequencies due to surface stresses as compared to the corresponding bulk material which does not observe surface effects within a finite deformation framework depending on whether fixed/free or fixed/fixed boundary conditions are utilized. We find that while the resonant frequencies of fixed/fixed nanowires are elevated as compared to the corresponding bulk material, the resonant frequencies of fixed/free nanowires are reduced as a result of compressive strain caused by the surface stresses. Furthermore, we find that for a diverse range of nanowire geometries, the variation in resonant frequencies for both boundary conditions due to surface stresses is a geometric effect that is characterized by the nanowire aspect ratio. The present results are found to agree well with existing experimental data for both types of boundary conditions. The second major goal of this work is to quantify, for the first time, how both the residual (strain-independent) and surface elastic (strain-dependent) parts of the surface stress impact the resonant frequencies of metal nanowires within the framework of nonlinear, finite deformation kinematics. We find that if finite deformation kinematics are considered, the strain-independent surface stress substantially alters the resonant frequencies of the nanowires; however, we also find that the strain-dependent surface stress has a significant effect, one that can be comparable to or even larger than the effect of the strain-independent surface stress depending on the boundary condition, in shifting the resonant frequencies of the nanowires as compared to the bulk material. & 2008 Elsevier Ltd. All rights reserved.. Introduction Over the past decade, nanowires, both metallic and semiconducting, have drawn considerable interest from the scientific community (Xia et al., 2003; Lieber, 2003). The large interest in nanowires has largely been driven by their Corresponding author. Tel.: ; fax: address: harold.park@colorado.edu (H.S. Park) /$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:0.06/j.jmps

3 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) remarkable physical properties, most of which emerge due to their small size and thus large surface area to volume (SAV) ratio. These properties range across the scientific disciplines, including unusual or enhanced optical (Canham, 990; Barnes et al., 2003), electrical (Wiley et al., 2006; Rubio et al., 996; Ohnishi et al., 998), thermal (Li et al., 2003) and mechanical (Wong et al., 997; Cuenot et al., 2004; Wu et al., 2005; Jing et al., 2006) properties. Nanowires are also important as they will serve as the basic building blocks for future nanoelectromechanical systems (NEMS), which have been proposed for a multitude of cross-disciplinary applications, including chemical and biological sensing, force and pressure sensing, high frequency resonators, and many others (Cleland and Roukes, 996; Huang et al., 2003; Craighead, 2000; Lavrik et al., 2004; Ekinci and Roukes, 2005; Ekinci, 2005). Because many of the proposed applications for nanowire-based NEMS, such as resonant mass sensing and high frequency oscillators (Craighead, 2000; Lavrik et al., 2004; Ekinci and Roukes, 2005) rely on the ability to control and tailor the nanowire resonant frequencies with a high degree of precision, it is critical to be able to predict and control variations in the nanowire resonant frequencies. The potential of nanowires in future nanotechnologies has led to significant interest in experimental characterization of the size-dependent elastic properties of nanowires. The experimental techniques utilized have varied from time-resolved spectroscopy (Petrova et al., 2006) to AFM-induced bending (Wong et al., 997; Wu et al., 2005; Heidelberg et al., 2006; Cuenot et al., 2004; Jing et al., 2006; Hoffmann et al., 2006; Chen et al., 2006; Namazu et al., 2000; Sundararajan et al., 2002) or resonance measurements (Verbridge et al., 2006, 2007; Cleland and Roukes, 996; Husain et al., 2003; Nam et al., 2006; Dikin et al., 2003; Yang et al., 200; Houston et al., 2002; Evoy et al., 2000). In general, resonance measurements to obtain the nanoscale elastic properties are predominant in the literature due to their relative simplicity as compared to bending and tensile experiments at the nanoscale due to the reduced amount of nanowire manipulation involved in resonance-based testing. The experimental results show significant scatter, with some predictions of enhanced elastic stiffness (Husain et al., 2003; Cuenot et al., 2004; Jing et al., 2006), some predicting reduced elastic stiffness (Petrova et al., 2006) with decreasing nanostructure size, and some predicting no change with respect to the bulk elastic stiffness (Wu et al., 2005; Heidelberg et al., 2006). The difficulty in predicting the resonant properties of nanowires stems from the fact that they are characterized by a large SAV ratio; because of this, nanowires are subject to surface stresses (Cammarata, 994; Haiss, 200), which occur due to the fact that surface atoms have fewer bonding neighbors than do atoms that lie within the material bulk. Surface stresses have been predicted to cause many non-bulk phenomena in nanowires, including self-healing behavior and phase transformations (Diao et al., 2003; Park et al., 2005; Liang et al., 2005b), and non-bulk elastic properties (Zhou and Huang, 2004; Liang et al., 2005a; Dingreville et al., 2005; Cuenot et al., 2004; Jing et al., 2006; Shenoy, 2005). The knowledge that surface effects are critical to understanding the mechanical behavior and properties of nanomaterials has motivated the development of enhanced continuum models, as standard continuum mechanics is length scale independent. Various analytic models have been developed to study the effects of surface stress on the resonant properties of nanobeams (Lu et al., 2005; Gurtin et al., 976; Sader, 200; McFarland et al., 2005), or more generally to capture the non-bulk mechanical properties of nanostructures (Gurtin and Murdoch, 975; Miller and Shenoy, 2000; Shenoy, 2005; Sharma et al., 2003; Sun and Zhang, 2003; Dingreville et al., 2005; Wei et al., 2006; Wang et al., 2006; Tang et al., 2006; Lu et al., 2005; Gurtin et al., 976; Sader, 200; Huang et al., 2006; McFarland et al., 2005). Due to assumptions utilized to make the analyses tractable, the coupled effects of geometry, surface orientation and system size on the resonant properties of nanowires have not been quantified, nor have surface stress effects arising directly from atomistic principles been included in the analyses, which are generally in two-dimensions. The analyses also utilize overly simplistic pair-type atomic interactions to describe the surface physics, which tend to incorrectly predict a compressive surface stress for metals, whereas the surface stress for metals is almost always tensile. These errors indicate that quantitative analyses for real materials cannot be made using these approaches. There are two major goals to the present work. The first is to quantify, for the first time, how surface stresses may be expected to alter the resonant frequencies for gold nanowires with a h00i axial orientation and f00g transverse surfaces considering both fixed/fixed and fixed/free boundary conditions as compared to the corresponding bulk material that does not observe nanoscale surface stress effects. These boundary conditions are ubiquitous in the study of NEMS, as most NEMS employ nanomaterials such as nanowires and nanotubes as the active beam element. We obtain the resonant frequencies using the recently developed surface Cauchy Born (SCB) model (Park et al., 2006; Park and Klein, 2007, 2008; Park, 2008a, b). The uniqueness of the SCB approach as compared to other analytical and theoretical (Gurtin and Murdoch, 975; Miller and Shenoy, 2000; Shenoy, 2005; Sharma et al., 2003; Sun and Zhang, 2003; Dingreville et al., 2005; Wei et al., 2006; Wang et al., 2006; Tang et al., 2006; Lu et al., 2005; Gurtin et al., 976; Sader, 200; Huang et al., 2006; McFarland et al., 2005) surface elastic models is that it enables the solution of three-dimensional nanomechanical boundary value problems for displacements, stresses and strains in nanomaterials using standard nonlinear finite element (FE) techniques (Belytschko et al., 2002), with the nonlinear, finite deformation material constitutive response obtained directly from realistic interatomic potentials such as the embedded atom method (EAM) (Daw and Baskes, 984). Furthermore, the usage of a standard FE formulation enables the consideration of arbitrary geometries and various materials once the SCB model has been developed. Therefore, the resonant properties of the gold nanowires are determined by solving a standard FE eigenvalue problem for the resonant frequencies and associated mode shapes, with full accounting for surface stress effects through the FE stiffness matrix. The present analysis does not account for factors that are known to deleteriously impact the resonant properties of nanostructures, including clamping losses and thermoelastic damping (Ekinci et al., 2004; Cleland and

4 346 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Roukes, 2002; Evoy et al., 999; Ilic et al., 2004; Yasumura et al., 2000; Carr et al., 999; Yang et al., 2000). We quantify the effects of surface stress on the fundamental resonant frequency as well as the higher order resonant frequencies corresponding to the deformation modes of second bending, twist and stretch for both fixed/free and fixed/fixed boundary conditions as functions of geometry, size and SAV ratio. We further compare the results to those obtained using the standard bulk Cauchy Born (BCB) material, which does not account for surface stress effects, while making contact with existing experimental data on the resonant frequencies and elastic properties of metal nanowires. The second major goal of the present work is to, for the first time within the framework of nonlinear, finite deformation kinematics, determine the effects of both the residual (strain-independent) and surface elastic (strain-dependent) parts of the surface stress on the resonant frequencies of metal nanowires. Since the work of Gurtin et al. (976), researchers have consistently found that, within the confines of linear elastic continuum beam theory, the strain-independent surface stress has no effect on the resonant frequency of a cantilever beam. However, Huang and Sun (2007) have recently demonstrated that if nonlinear, finite deformation kinematics are considered, that the residual surface stress does in fact change the effective elastic properties of the nanostructure. We therefore quantify in this work the effects of the residual and surface elastic parts of the surface stress on the resonant properties of metal nanowires using a modified version of the SCB model in which the strain-dependent parts of the surface stress and surface stiffness are subtracted from the original, fully nonlinear SCB model. 2. SCB model 2.. Overview Details regarding the SCB model and its differences from the standard BCB model have been described in previous publications (Park et al., 2006; Park and Klein, 2007, 2008; Park, 2008a, b). Therefore, we briefly overview the main ideas of the SCB model here. The BCB model is a hierarchical multiscale assumption that enables the calculation of continuum stress and moduli from atomistic principles (Tadmor et al., 996). Because the BCB model does not consider surface effects, the SCB model was developed (Park et al., 2006; Park and Klein, 2007, 2008) such that the energy density of a material would include contributions not only from the bulk, but also the material surfaces thus leading to the incorporation of atomistic-based surface stress effects into standard continuum stress measures. Both the BCB and SCB models are finite deformation constitutive models that explicitly represent the stretching and rotation of bonds undergoing large deformation through continuum mechanics-based kinematic quantities such as the deformation gradient F, or the stretch tensor C ¼ F T F (Belytschko et al., 2002). Under deformations which can be represented as homogeneous over the unit cell scale, the approximation exactly reproduces the response of the corresponding, fully atomistic representation of the crystal. The necessity for the finite deformation kinematics gains credence through recent work that has indicated that surface stresses can cause nonlinearly elastic compressive strains on the order of % or more in the nanowires (Park and Klein, 2007; Liang et al., 2005a). The finite deformation formulation utilized for the SCB model also enables the calculation of resonant frequencies of highly deformed nanostructures, which may be useful due to the large elastic deformations that nanowires can undergo prior to yield and subsequent failure and due to the numerous sensing applications that are envisioned utilizing nanowires as the sensing component (Ekinci and Roukes, 2005; Park, 2008a). A schematic of the SCB decomposition of bulk/non-bulk atoms near a free surface is shown in Fig. ; note that all atomic interactions involving bulk and non-bulk atoms are governed entirely by the range of the interatomic potential chosen, as would be in an atomistic simulation. Mathematically, the relationship between the continuum strain energy density and the total potential energy of the corresponding, defect-free atomistic system can be written as natoms X i Z U i ðrþ ¼ O bulk 0 Z FðCÞ do þ G 0 Z g G ðcþ dg þ 0 G 2 0 Z g G 2ðCÞ dg þ 0 G 3 0 Z g G 3 ðcþ dg þ 0 G 4 0 g G 4ðCÞ dg, () 0 Fig.. Illustration of bulk and non-bulk layers of atoms in a h 0 0i=f 0 0g FCC crystal interacting by an EAM potential.

5 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) where U i is the potential energy for atom i, r is the interatomic distance, FðCÞ is the bulk strain energy density, O bulk 0 ðcþ is the surface strain energy density of a represents the volume of the body in which all atoms are fully coordinated, g G a 0 representative atom in surface layer a and natoms is the total number of atoms in the system. The bulk strain energy density FðCÞ in this work is obtained using EAM potentials (Daw and Baskes, 984; Foiles et al., 986), and takes the form FðCÞ ¼ O 0 ðf i ð r i Þþf i Þ, (2) f i ¼ 2 Xnbrv i jai f ij ðr ij ðcþþ, (3) r i ¼ Xnbrv i r j ðr ij ðcþþ, (4) jai where nbrv i are the number of bonds in the representative unit volume O 0 for atom i, F i is the embedding function, r j is the contribution to the electron density at atom i from atom j, f ij is a pair interaction function and r ij is the distance between atoms i and j. Analogous to the bulk energy density, the surface energy densities gðcþ describe the energy per representative undeformed area of atoms at or near the surface of a homogeneously deforming crystal, and is also obtained using the same EAM potential as the bulk energy density. For FCC metals, choosing a surface unit cell that contains only one atom is sufficient to reproduce the structure of each surface layer. The surface unit cell possesses translational symmetry only in the plane of the surface, unlike the bulk unit cell which possesses translational symmetry in all directions. Thus, the surface energy density g G a ðcþ for a representative atom in a given surface layer Ga 0 0 in Fig. can be written as g G a 0 ðcþ ¼ G 0 ðf i ð r i Þþf i Þ, (5) f i ¼ 2 X nba jai f ij ðr ij ðcþþ, (6) r i ¼ Xnba r j ðr ij ðcþþ, (7) jai where nb a are the number of bonds for an atom in surface layer a, and G 0 is the representative unit area occupied by a non-bulk atom lying at or near the free surface. Once the bulk strain energy density is known, continuum stress measures such as the second Piola Kirchoff stress S can be defined as S ¼ 2 qfðcþ qc, (8) while the material tangent modulus C is defined to be C ¼ 2 qs qc. (9) Similarly, the surface stress on each surface layer G a 0 in Fig. can be defined as ~S ðaþ qg G ðcþ ¼2 a ðcþ 0, (0) qc while the surface tangent modulus C ~ can be written as ~C ¼ 2 q~ S qc. () The SCB model thus uses the surface unit cells based on the surface energies gðcþ to capture the undercoordination of atoms in the surface layers. Because the surface unit cells are undercoordinated, they are not at a minimum energy at the same atomistic spacing as bulk atoms, which results in the existence of surface stresses in Eq. (0) through differentiation of the surface energies gðcþ. It is critical to emphasize again that both the surface stress in Eq. (0) and the surface tangent modulus in Eq. () are quantities based upon finite deformation kinematics that are functions of strain through the stretch tensor C. We also discuss here differences between the current formulation for surface stress and surface energy, and the traditional thermodynamic definition of surface stress (see for example Cammarata, 994; Shenoy, 2005): t ¼ t 0 þ C 0 e, (2)

6 348 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) where t is the surface stress, t 0 is the residual (strain-independent) portion of the surface stress, C 0 e is the surface-elastic (strain-dependent) part of the surface stress, and where C 0 is the (constant) surface elastic stiffness. We will return to this definition of surface stress later when investigating the effects of the strain-independent part of the surface stress on the nanowire resonant frequencies. The thermodynamic interpretation of both the surface stress t in Eq. (2) and that of the surface energy g G a ðcþ in Eq. (5) 0 is that of an excess quantity, i.e. a measure of the difference as compared to the equivalent bulk quantity. The surface energy g G a ðcþ in Eq. (5) differs from the conventional definition in that it does not represent the excess, or difference in 0 surface energy as compared to a typical bulk atom; instead, it represents the actual potential energy of an atom lying in surface layer a. Furthermore, the definition of surface stress utilized in the present work in Eq. (0) differs from Eq. (2) in that the surface energy is directly differentiated in the present work to obtain the surface stress in Eq. (0). This choice can be understood by analyzing the energy balance in Eq. (). Because Eq. () represents the total energy of the nanostructure as decomposed into bulk and surface contributions, minimization of the energy leads directly to a force balance (Park et al., 2006; Huang and Wang, 2006), which carries the clear physical meaning that at equilibrium, the bulk forces will balance the surface forces that originate from the surface stress. Furthermore, starting from the energy balance in Eq. () is extremely favorable for nonlinear FE implementation; the full details of the FE equations are found in Park et al. (2006).We note in closing that an extensive analysis of the SCB model in calculating the minimum energy configurations of gold nanowires as compared to benchmark atomistic calculations can be found in Park and Klein (2007) FE eigenvalue problem for nanowire resonant frequencies The equation describing the eigenvalue problem for continuum elastodynamics is written as ðk o 2 MÞu ¼ 0, (3) where M is the mass matrix and K is the stiffness matrix of the discretized FE equations; the solution of the eigenvalue problem described in Eq. (3) gives the resonant frequencies f, where f ¼ o=2p and the corresponding mode shapes u. We note that the stiffness matrix K contains the effects of both material and geometric nonlinearities through a consistent linearization about the finitely deformed configuration (Belytschko et al., 2002). As detailed in Park and Klein (2007), once the total energy is obtained by subtracting from Eq. () the work due to external loads, the FE equilibrium equations can be obtained by approximating the displacement field using standard FE interpolation functions (Belytschko et al., 2002) and taking the first variation of the total energy. We emphasize that the addition of the surface energy terms in Eq. () leads naturally to the incorporation of the surface stresses in the FE stiffness matrix K, which then leads to the dependence of the resonant frequencies f on the surface stresses. The eigenvalue problem was solved using the Sandia-developed package Trilinos, which was incorporated into the simulation code Tahoe. 3. Numerical examples All numerical examples were performed on three-dimensional, single crystal gold nanowires that have a cross-section of width a and length h as illustrated in Fig. 2. Three different parametric studies are conducted in this work, which consider nanowires with constant cross-sectional area (CSA), constant length and constant SAV; the geometries are summarized in Table. All wires had a h 0 0i longitudinal orientation with f 0 0g transverse surfaces, and had either fixed/free (cantilevered) boundary conditions, where the left ð xþ surface of the wire was fixed while the right ðþxþ surface of the wire was free, or fixed/fixed boundary conditions, where both the left ð xþ and right ðþxþ surfaces of the wire were fixed. All FE simulations were performed using the stated boundary conditions without external loading, and utilized regular meshes of 8-node hexahedral elements. The SCB bulk and surface energy densities in Eqs. (2) and (5) were calculated using EAM interatomic potentials, with gold being the material for all problems using the parameters of Foiles et al. (986). In the present work, the bulk FE stresses were calculated using Eq. (8) and the surface FE stresses were found using Eq. (0). We note also that the nanowire cross-sections considered in this work are sufficiently large such that the nanowire surfaces do not reconstruct or reorient to a lower energy orientation; it is well-established (Kondo et al., 999; Hasmy and Medina, 2002) that gold f00g surfaces will reorient to lower energy fg surfaces only if the thin film or nanowire Fig. 2. Nanowire geometry considered for numerical examples.

7 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Table Summary of geometries considered: constant surface area to volume ratio (SAV), constant length, and constant cross-sectional area (CSA) Constant SAV Constant length Constant CSA :7 9: :2 5:2 232 :6 : :9 4: :95 5: :7 4: :2 23: :5 4:5 All dimensions are in nm. thickness is less than about 2 nm. Thus, the f00g nanowire surfaces in this work undergo only nonlinear elastic deformations. Regardless of boundary condition, the nanowires are initially out of equilibrium due to the presence of the surface stresses. For fixed/free nanowires, the free end undergoes a compressive relaxation strain to find an energy minimizing configuration under the influence of surface stresses; previous work (Park and Klein, 2007) illustrated the accuracy of the SCB model in predicting energy minimizing configurations of the nanowires due to surface stresses. Fixed/fixed nanowires, on the other hand, are constrained such that the nanowire free surfaces are unable to contract due to the boundary conditions. Therefore, fixed/fixed nanowires exist in a state of tension, as they are unable to contract despite the presence of the surface stresses. For both boundary conditions, the minimum energy configuration was obtained while accounting for the surface stresses. The fixed/fixed boundary conditions utilized in this work represent nanowires that are fabricated through a top-down process of etching or lifting-off of a metal film on a semiconducting substrate (Davis and Boisen, 2005; Li et al., 2003, 2007) as is commonly done in experimental studies of NEMS-based nanowire resonance. Experimentally, the residual tension in the fixed/fixed nanowires would occur due to surface stress effects on the nanowires that are etched in a fixed configuration, and are therefore unable to contract axially to relieve the tensile surface stresses. At that point, the eigenvalue problem described in Eq. (3) is solved using the FE stiffness matrix from the equilibrated (deformed) nanowire configuration to find the resonant frequencies. Resonant frequencies were also found using the standard BCB model (without surface stresses) on the same geometries for comparison. For all resonant frequencies reported in this work, the fundamental, or lowest mode frequencies corresponded to a standard bending mode of deformation. For each nanowire geometry, we plot the resonant frequencies in two ways. First, we plot the normalized resonant frequency f scb =f bcb versus the aspect ratio h=a. Second, we plot the normalized resonant frequency f scb =f bcb versus the SAV ratio to quantify the resonant frequency variation that the surface stresses cause. Finally, we compare the results for the fixed/free nanowires to those of the fixed/fixed nanowires to ascertain the effects of boundary conditions and surface stresses on the predicted resonant frequencies. 3.. Constant CSA For the first set of simulations, nanowires with constant square cross-section of width a ¼ 6 nm and increasing length h were considered; the lengths h considered ranged between four and 24 times a, resulting in FE meshes that contained between 5000 and nodes. To validate the accuracy of the calculations for the bulk material, we compare in Tables 2 and 3 the BCB and SCB resonant frequencies to those obtained using the well-known analytic solution for the fundamental resonant frequency for both fixed/free (cantilevered) and fixed/fixed beams (Weaver et al., 990). For the fixed/free beam: sffiffiffiffiffiffi EI f 0 ¼ B2 0, (4) 2ph 2 ra where B 0 ¼ :875 for the fundamental resonant mode and E is the modulus in the h00i direction, which can be found to be 35 GPa (Foiles et al., 986). The BCB resonant frequencies compare quite well to those predicted by the analytic formula, with increasing accuracy for increasing aspect ratio h=a, as would be expected from beam theory. We note that the SCB resonant frequencies become smaller than the bulk resonant frequencies when the aspect ratio h=a48; reasons for this trend, which will be observed in all parametric studies, will be discussed later. For the fixed/fixed beam, the analytic solution is given as (Weaver et al., 990) sffiffiffiffiffiffi f 0 ¼ i2 p EI, (5) 2h 2 ra where i is a mode shape factor, which is about.5 for the fundamental bending mode of fixed/fixed beams with zero displacement and zero slope boundary conditions (Verbridge et al., 2006; Weaver et al., 990). Table 3 shows that the bulk

8 350 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Table 2 Summary of constant CSA nanowire fundamental resonant frequencies for fixed/free boundary conditions as computed from: () the analytic solution given by Eq. (4), (2) bulk Cauchy Born (BCB), and (3) surface Cauchy Born (SCB) Geometry Eq. (4) BCB SCB All frequencies are in MHz, the nanowire dimensions are in nm. Table 3 Summary of constant CSA nanowire fundamental resonant frequencies for fixed/fixed boundary conditions as computed from: () the analytic solution given by Eq. (5), (2) bulk Cauchy Born (BCB), and (3) surface Cauchy Born (SCB) Geometry Eq. (5) BCB SCB All frequencies are in MHz, the nanowire geometry is in nm..5.4 Constant CSA Nanowires, a = 6 nm Fixed/free Fixed/fixed Fig. 3. Variation in fundamental resonant frequency for fixed/free and fixed/fixed constant cross-sectional area nanowires as a function of nanowire aspect ratio h=a. CB and analytic frequencies agree nicely, while the SCB resonant frequencies are consistently higher; the reasons for this will be discussed in detail below. Fig. 3 shows the resonant frequencies for both the fixed/fixed and fixed/free cases plotted as a function of the aspect ratio h=a. As can be seen, the calculated resonant frequencies vary greatly depending on the boundary condition. For small aspect ratios, both boundary conditions give resonant frequencies close to the bulk value. However, as the aspect ratio h=a increases, the fixed/free resonant frequencies decrease relative to the bulk value, while the fixed/fixed resonant frequencies increase relative to the bulk value. For the fixed/free case calculated in this work, the resonant frequency is reduced to nearly 90% of the bulk value when the aspect ratio reaches h=a ¼ 24, while the resonant frequency for fixed/fixed nanowires increase to more than 40% of the bulk value when h=a ¼ 24. The results can also be analyzed with respect to the SAV ratio, as seen in Fig. 4. We note that in this case, increasing the length while keeping the CSA constant leads to a decrease in SAV ratio. Again, the boundary conditions impact the trends with SAV ratio; while the fixed/fixed nanowires show a decrease in resonant frequency with increasing SAV ratio, the opposite is observed for the fixed/free nanowires.

9 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Constant CSA Nanowires, a = 6 nm.4 Fixed/free Fixed/fixed Surface area to volume ratio (nm ) Fig. 4. Variation in fundamental resonant frequency for fixed/free and fixed/fixed constant cross-sectional area nanowires as a function of surface area to volume ratio Constant length Simulations were also performed keeping the length of the nanowire fixed at h ¼ 232 nm, while increasing the CSA. Aspect ratios of h=a ¼ 0 24 were studied to quantify the variation in resonant frequencies for the two boundary conditions; the resulting FE meshes ranged in size from about to nodes. The normalized resonant frequencies are plotted versus aspect ratio h=a in Fig. 5. Again, the fixed/fixed nanowires show a marked increase in resonant frequency with increasing h=a, while the fixed/free nanowires show a marked decrease in resonant frequency with h=a. For the smallest nanowires considered in this work, with a transverse dimension of 9.7 nm, the fixed/free resonant frequency is only 90% of the bulk value, while the fixed/fixed resonant frequency is more than 60% of the bulk value. In all cases, the fixed/free resonant frequencies are below the bulk value. An interesting result is obtained when the resonant frequencies for the constant length geometry are plotted against SAV ratio in Fig. 6. Unlike the constant CSA resonant frequencies shown in Fig. 4, the fixed/free resonant frequencies decrease with increasing SAV ratio, while the fixed/fixed resonant frequencies increase with increasing SAV ratio Constant SAV ratio To draw general conclusions about the impact of SAV ratio on the nanowire resonant frequencies, we calculate the resonant frequencies of nanowires that have the same SAV ratio ð0:28 nm Þ, but different square cross-sections of length a and longitudinal length h; a ranged from 4.5 to 6 nm, while h ranged from 64 to 290 nm, leading to FE mesh sizes ranging from about 5000 to nodes. The results for the constant SAV ratio nanowires are shown in Fig. 7; in this case, because the SAV ratio is constant, we plot the resonant frequencies for both boundary conditions only with respect to the aspect ratio h=a. Fig. 7 clearly shows that the resonant frequency does not remain constant if the SAV ratio remains constant, for either fixed/free or fixed/fixed boundary conditions. In fact, the results strongly mirror those presented earlier in Fig. 3 for the constant CSA nanowires, and thus indicate that the nanowire aspect ratio h=a is a much more reliable tool to controlling, predicting and tailoring the resonant frequencies of nanowires than is the SAV ratio. This fact will be discussed later in this work Higher order modes In addition to analyzing boundary condition and surface stress effects on the fundamental resonant frequencies of gold nanowires, we also now analyze their effects on the higher order resonant frequencies corresponding to the modes of second bending, twist and stretch. We first plot the higher order mode resonant frequencies normalized by the corresponding bulk values of each mode for the two boundary conditions and for the constant CSA nanowires. The fixed/fixed higher order modes are plotted in Fig. 8, while the fixed/free higher order modes are plotted in Fig. 9. In both cases, the largest variation occurs in the second

10 352 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Constant Length Nanowires, h = 232 nm Fixed/free Fixed/fixed Fig. 5. Variation in fundamental resonant frequency for fixed/free and fixed/fixed constant length nanowires as a function of nanowire aspect ratio h=a..8.7 Constant Length Nanowires, h = 232 nm Fixed/free Fixed/fixed Surface area to volume ratio (nm ) Fig. 6. Variation in fundamental resonant frequency for fixed/free and fixed/fixed constant length nanowires as a function of surface area to volume ratio. bending mode resonant frequency, which increases with h=a for the fixed/fixed case similar to the fundamental mode resonant frequency, and decreases with h=a for the fixed/free case, again similar to the fundamental mode resonant frequency. In contrast, the twist and stretch frequencies show little variation with increasing aspect ratio h=a. We also plot in Figs. 0 and the variation in the higher mode resonant frequencies by normalizing each mode resonant frequency for both the BCB and SCB calculations by the first (fundamental) bending mode resonant frequency ðf scb scb stretch =f bend ; f bulk bulk stretch =f bend ; etc.þ, to quantify how surface stresses cause the higher mode (second bending, stretch, twist) resonant frequencies to either increase or decrease relative to the fundamental bending resonant frequency. Figs. 0 and illustrate two points: first, how the higher mode (second bending, twist, stretch) resonant frequencies change with respect to the fundamental resonant frequency with increasing h=a, and second, how surface stresses alter the changes as compared to the BCB material. These figures again illustrate the effects of boundary conditions and surface stresses on the resonant frequencies. For example, in the fixed/fixed case in Fig. 0, all higher mode resonant frequencies decrease due to the surface stresses, with the effects being particularly dramatic for the stretch and twist modes; the f stretch =f bend ratio including surface stresses is only 7% of the equivalent bulk ratio when h=a ¼ 24, while the twist differential is nearly identical. Similarly, the f bend2 =f bend ratio including surface stresses is about 87% of the bulk ratio when h=a ¼ 24.

11 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Fixed/free Fixed/fixed Constant SAV Nanowires Fig. 7. Variation in fundamental resonant frequency for fixed/free and fixed/fixed constant surface area to volume ratio nanowires as a function of nanowire aspect ratio h=a (a) Fixed/Fixed Constant CSA Nanowires, a = 6 nm Second Bending Stretch Twist Fig. 8. Surface stress effects on higher mode resonant frequencies for fixed/fixed nanowires with constant cross-sectional area. The trends for the fixed/free nanowires are completely different, as shown in Fig.. There, all higher mode resonant frequencies increase due to the surface stresses relative to the fundamental resonant frequency. The f stretch =f bend ratio including surface stresses is 08% of the equivalent bulk ratio when h=a ¼ 24, while f twist =f bend is 6% of the equivalent bulk ratio. Similarly, the f bend2 =f bend ratio including surface stresses is about 09% of the bulk ratio when h=a ¼ Discussion, analysis and comparison to existing theoretical and experimental studies 4.. Fixed/fixed nanowires We now turn our attention to the fact that the resonant frequencies of fixed/fixed nanowires due to surface stresses as predicted using the SCB model are consistently larger than the resonant frequencies of the corresponding bulk material. Experimental measurements of the elastic properties of metal nanowires have generally focused upon using the atomic force microscope (AFM) to calculate the elastic properties through bending related techniques (Wu et al., 2005; Heidelberg

12 354 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) (a) Fixed/Free Constant CSA Nanowires, a = 6 nm Second Bending Stretch Twist Fig. 9. Surface stress effects on higher mode resonant frequencies for fixed/free nanowires with constant cross-sectional area. Log 0 (f/f bend ) Fixed/Fixed Constant CSA Nanowires Bulk 2nd Bending SCB 2nd Bending Bulk Stretch SCB Stretch Bulk Twist SCB Twist Fig. 0. Surface stress effects on higher mode resonant frequencies relative to the fundamental bending frequency for fixed/free nanowires with constant cross-sectional area. et al., 2006; Jing et al., 2006). Alternatively, some researchers have utilized resonance-based measurements to extract the elastic properties. One such study was performed by Cuenot et al. (2004), who utilized electrostatic resonant-contact AFM to determine Young s modulus of silver and lead nanowires by first calculating the resonant frequencies. The nanowires analyzed in that work had fixed/fixed boundary conditions, while the diameters ranged from about 50 to 200 nm. The major finding of the Cuenot et al. work is that the nanowire Young s modulus (and therefore the resonant frequency) increases with respect to the bulk value with a reduction in CSA. Similar results were obtained by Jing et al. (2006), who utilized an AFM to perform three-point bend tests to extract the elastic properties of fixed/fixed silver nanowires. For nanowires with diameters ranging from 20 to 40 nm, similar results to those of Cuenot et al. were obtained, i.e. an increasing modulus with decreasing size. Due to the existence of an analytic solution that was derived by Cuenot et al. (2004) to model the observed increase in nanowire Young s modulus with decreasing size for the fixed/fixed boundary conditions, we compare results obtained in the present work with their analytic solution. From the work of Cuenot et al. (2004), the analytic solution is E scb ¼ E bulk þ 8 h2 ð nþg 5 D, (6) 3

13 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Fixed/Free Constant CSA Nanowires Log 0 (f/f bend ) Bulk 2nd Bending SCB 2nd Bending Bulk Stretch SCB Stretch Bulk Twist SCB Twist Fig.. Surface stress effects on higher mode resonant frequencies relative to the fundamental bending frequency for fixed/free nanowires with constant cross-sectional area Fixed/Fixed Constant CSA Nanowires, a=6 nm Cuenot et al. (2004), Equation (9) Resonance Formula Ratio E scb /E bulk Fig. 2. E scb =E bulk as a function of aspect ratio h=a due to surface stress effects for fixed/fixed constant CSA nanowires. where h is the nanowire length, E scb is the apparent Young s modulus taking surface effects into account, E bulk is the bulk Young s modulus, D is the nanowire diameter, g is the surface energy and n is Poisson s ratio. We note that Eq. (6) implies that the modulus of the nanowires will change both as functions of the length, cross-sectional diameter, and therefore the aspect ratio. Here, the relaxed f00g surface energy g ¼ 0:94 J=m 2 for the Foiles et al. (986) potential was utilized, while the bulk Poisson s ratio of 0.44 for gold was utilized. We also note that in the Cuenot et al. (2004) work, the analytic relation in Eq. (6) was not actually utilized to compare against the experimentally obtained results. Fig. 2 thus shows the E scb =E bulk ratio for the present work, which is obtained by substituting the numerically obtained resonant frequencies for both the SCB and BCB cases into Eq. (5) to solve for the SCB and BCB Young s moduli. We utilize the standard continuum beam theory equation in Eq. (5) to solve for the SCB Young s modulus due to the fact that the analytic solution given in Eq. (6) is also based upon continuum beam theory. As can be seen, the modulus ratio obtained through the present work agrees well with the analytic solution of Cuenot et al. (2004) for the aspect ratios considered in this work, and illustrates that the expected length dependence of the nanowire Young s modulus is captured in the present work. The present results are not expected to agree exactly with the analytic solution in Eq. (6) for multiple reasons; the major reasons include the fact that metal nanowires typically form

14 356 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) oxide layers on the surface (Jing et al., 2006; Cuenot et al., 2004), and also because the SCB resonant frequencies were calculated using finite deformation kinematics, where the stiffness of the nanowire changes due to deformation induced by surface stresses. In contrast, the Cuenot et al. (2004) beam theory solution is based upon linear elasticity, and therefore constant material properties for both the bulk and surface components. Despite these uncertainties, the general trend in Fig. 2 of increasing nanowire Young s modulus with increasing aspect ratio for fixed/fixed boundary conditions is correctly predicted using the SCB model. More importantly, Cuenot et al. (2004) experimentally observed an increase in apparent Young s modulus with decreasing size for fixed/fixed silver nanowires; the same trend is predicted in the present work for gold, and is explained naturally by understanding that the fixed/fixed nanowires exist in a state of tensile stress due to the fixed/fixed boundary conditions, which prevent surface stresses from contracting the nanowires. We also compare our results with those obtained from Husain et al. (2003); we make this comparison due to the fact that they also studied the resonant properties of a fixed/fixed FCC nanowire, in this case platinum. To compare with their larger nanowires (cross-section of 43 nm, length of 300 nm), we created gold nanowires of the same dimensions, and calculated the resonant frequency using the SCB model. The Husain et al. (2003) work found a resonant frequency for their fixed/fixed platinum nanowire of about 05 MHz, which exceeds the predicted value of 69 MHz as calculated using Eq. (5), with the experimentally measured resonant frequency being.5 times larger than the predicted resonant frequency. For gold, the predicted resonant frequency using Eq. (5) should be about 35 MHz; the SCB calculations predicted a resonant frequency of 47.5 MHz, thus leading to the SCB results being about.4 times larger than that expected using Eq. (5). In the Husain et al. (2003) work, thermal effects due to mismatch between the nanowire and the substrate were cited as being a likely cause for placing their platinum nanowire under tension. The present results indicate that surface stress effects are likely to have played a role in elevating the experimentally observed resonant frequencies above those predicted using bulk continuum measures. We also discuss the experiments of Verbridge et al. (2006), who studied the effect of applied tensile stress on the resonant frequencies of fixed/fixed SiN nanowires. This study is of relevance as it is one of the few to study the effects of tensile stresses on the resonant frequencies of nanowires, and thus is analogous to our situation where surface stresses cause a tensile stress within the gold nanowires. A key finding of that work was the fact that the resonant frequency was found to vary linearly with the inverse of length; as shown in Verbridge et al. (2006), the resonant frequency of a beam under high tensile stress can be written as sffiffiffiffiffiffi f ¼ i S, (7) 2h ra where S is the force in the beam. Interestingly, as can be seen in Fig. 3, both the SCB and BCB resonant frequencies scale linearly with =h 2, instead of =h. The bulk dependence on =h 2 is not surprising considering the =h 2 dependence of the analytic solution given in Eq. (5). However, the same dependence of the SCB resonant frequencies is interesting, and implies that the surface stresses, and the nonlinear elastic deformation they cause in the nanowire, may be responsible for the variation from the behavior observed experimentally in the larger (00 nm) SiN nanowires. We also note that the different surface stresses of metallic (generally tensile Wan et al., 999) and semiconductor (sometimes compressive Balamane et al., 992) surfaces may also help to explain the observed differences. Finally, for the nanowires considered in that work, it is likely that the 00 nm cross-section is too large for surface stresses to have a significant effect on the mechanical properties, and thus the measured resonant frequencies Fixed/free nanowires The large disparity in the calculated resonant frequencies between the fixed/free and fixed/fixed cases indicates that the nanowire stiffness and state of stress is strongly dependent on the boundary conditions. As previously discussed, the fixed/fixed boundary conditions prevent the nanowires from relaxing axially, as would occur due to surface stresses if one of the ends were free (Diao et al., 2003; Park et al., 2005; Liang et al., 2005b). Thus, the fixed/fixed nanowires exist in a state of tension, which elevates their resonant frequencies as compared to the unstrained fixed/fixed bulk material. However, when fixed/free boundary conditions are utilized, the free end of the nanowire is able to contract axially in response to the tensile surface stresses, thereby reducing their transverse surface area and finding a minimum energy configuration. The prediction of the fixed/free resonant frequencies is therefore dependent on obtaining the correct stiffness of the nanowires after the surface-stress-driven relaxation has occurred; previous work (Park and Klein, 2007) indicated the ability of the SCB model to predict the correct relaxation strain due to surface stresses. Fig. 4 illustrates one of the key findings of this work. In Fig. 4, we plot the variation in the fixed/free nanowire resonant frequencies due to surface stresses for all three geometries considered in this work (constant CSA, constant length, constant SAV ratio) versus the nanowire aspect ratio h=a. As can be seen, for the diverse geometries we have considered, the variations in resonant frequency due to surface stresses are quite similar as a function of h=a, indicating that the resonant frequency variation is purely geometric in nature.

15 H.S. Park, P.A. Klein / J. Mech. Phys. Solids 56 (2008) Fixed/Fixed Constant CSA Nanowires, a = 6 nm Fundamental Bending Bulk Fundamental Bending SCB 0.25 f/f bend /h 2 (/nm 2 ) x 0 5 Fig. 3. Linear variation of both SCB and bulk CB resonant frequencies for constant CSA nanowires as a function of =h Constant SAV Constant Length Constant CSA Nanowire aspect ratio Fig. 4. Relationship between nanowire aspect ratio h=a and fundamental resonant frequency shift f scb =f bulk observed by fixed/free nanowires of constant cross-sectional area, length, and surface area to volume ratio. Recent atomistic simulations (Liang et al., 2005a; Diao et al., 2006) have indicated that the nanowire elastic properties are strongly dependent upon the amount of compressive surface-stress-driven relaxation strain that the nanowires undergo. We therefore plot in Fig. 5 the variation in nanowire resonant frequencies for the three different geometry types as a function of the percent compressive relaxation strain. As can be observed, while the nanowire resonant frequencies do decrease with increasing relaxation strain, the variation is not the same for the three geometries, which indicates that knowledge of the state of strain is not sufficient to predict the resonant frequencies, and therefore the elastic properties of the fixed/free nanowires. Furthermore, this indicates that the nanowire aspect ratio h=a is the critical geometric parameter for determining the resonant frequency variations due to surface stresses as compared to the corresponding bulk material. The reduction in resonant frequencies for the fixed/free h 0 0i nanowires considered in this work is due to the fact that bulk FCC metals tend to soften if compressed in the h00i direction (Liang et al., 2005a); in the present work, the compression arises due to the surface stresses, rather than any externally applied forces. Therefore, the resonant frequencies for other orientations, for example the h0i orientation in which FCC metals are known to stiffen under compression (Liang et al., 2005a), may increase rather than decrease for fixed/free boundary conditions as compared to the bulk material.